This invention relates to a flue gas desulfurization process that utilizes a sodium sulfite- and sodium bisulfite-containing alkaline absorption solution regenerated with limestone.
Flue gas desulfurization processes that employ alkaline absorption solutions containing both sodium sulfite and sodium bisulfite for removing sulfur oxides like SO.sub.2 from flue gases are well known and are normally operated continuously with the spent absorption solution being regenerated with lime or limestone. Such lime/limestone-sodium salt processes are often termed "double" or "dual" alkali processes in contradistinction to single alkali processes that directly treat the flue gas with lime or limestone.
Double alkali processes that utilize lime as the regenerating agent are in commercial use throughout the United States. U.S. Pat. No. 3,911,084 issued to Wall et al. discloses one such lime-sodium double alkali process.
Double alkali processes described in the patent literature which provide for the efficient utilization of lime, as the regenerating agent, often suggest that limestone may be substituted for lime. Such a substitution, however, is not at all straightforward, since operating factors such as desulfurization efficiency, regenerating agent utilization (reaction rate and reaction completion), and by-product solid physical characteristics are distinctly different for these two chemical regenerating agents.
It is instructive to note that despite definite cost advantages of limestone over lime, no limestone-sodium double alkali processes are in commercial use in the United States. This is apparently so because no efficient procedures have been devised for operating such processes competitively with lime-sodium double alkali processes.
Limestone-sodium double alkali processes that are described in the prior art provide for efficient absorption typically by sacrificing efficiency during the regeneration operation. U.S. Pat. No. 3,848,070 issued to Onozuka et al., No. 3,944,649 issued to Field et al. and No. 3,989,796 issued to Morita et al., teach that at least a stoichiometric amount of limestone should be employed in the complete neutralization of the bisulfite during regeneration. Such prior art processes, moreover, are generally operated with relatively high alkali sulfite/bisulfite concentrations in the absorption solution to maximize SO.sub.2 -absorption capacity, but this mode of operation requires significant alkali make-up to compensate for process solution losses.
The present invention provides a method for operating a limestone-sodium double alkali flue gas desulfurization process with superior efficiency in both the absorption and regeneration operations.